As transistor gate lengths continue to shrink, threshold voltages (V.sub.t) continue to decrease. Below 1.0 volts, it may become difficult to balance adequate current drive with acceptable leakage current. One way to address this problem is to design a device that has a dynamic threshold voltage, i.e., a device that has a lower V.sub.t when the device is turned on than when the device is turned off. By varying the threshold voltage in this way, even a low voltage device may provide a high drive current while continuing to maintain low subthreshold leakage.
Devices having a dynamic threshold voltage may be made by tying a body strap formed below the transistor's channel to the transistor's gate electrode, as described by Hu, et al., "A Dynamic Threshold Voltage MOSFET (DTMOS) for Ultra-Low Voltage Operation," Int'l Electron Devices Meeting Technical Digest, 1994, pp. 809-812; and Hu et al., "Channel Profile Optimization and Device Design for Low-Power High-Performance Dynamic-Threshold MOSFET," Int'l Electron Devices Meeting Technical Digest, 1996, pp. 113-116.
Such devices may use silicon-on-insulator ("SOI") technology to electrically isolate the body strap from the junctions and adjacent transistors. FIG. 1 illustrates such a device, which includes relatively thick silicon film 104 formed on oxide 103. As shown, body strap 102 is separated from junctions 181 and 111, which correspond to the lower boundaries for source 106 and drain 107, respectively. Because of that separation, the capacitance that may result, when the device is turned on, can slow down the rate at which the body potential increases in response to an increase in the voltage applied to gate electrode 105.
One way to address that problem is to use a relatively thin silicon layer instead of a relatively thick layer. FIG. 2 illustrates such a device, which includes relatively thin silicon film 204 formed on oxide 203. When a thin film is used, however, misalignment between body strap 202 and gate electrode 205 can cause a short circuit between body strap 202 and one of the junctions, e.g., junction 211 in the device shown here.
There is a need for a device that does not include such short circuits, or produce unacceptable capacitance that slows down the rate of body potential increase when the device is turned on. There is also a need for a method for making such a device.